The present invention concerns a 2-cycle engine furnished in an outdoor trimmer, backpack-type power spreader, or other such device, and the present invention specifically concerns a 2-cycle engine effecting a reduction in total hydrocarbons (THCs).
In a 2-cycle engine furnished in an outdoor trimmer, backpack-type power spreader, or other such devices, a fuel-containing gas consisting of fuel and air within a crankcase is introduced from a scavenging port into a combustion chamber during a scavenging stroke, scavenging of the combustion chamber continues, and the combustion chamber becomes filled. The objective of conventional 2-cycle engine design has been to increase crankcase pressure as much as possible in order to complete scavenging in a short time.
In a conventional 2-cycle engine, the fuel-containing gas introduced from the scavenging port into the combustion chamber does not stop in the combustion chamber; short-circuiting occurs wherein this gas escapes without modification into an exhaust port, and the fuel component within the short-circuited fuel-containing gas is released together with exhaust gases into the atmosphere and becomes a source of atmospheric pollution.
An object of the present invention is to offer a 2-cycle engine able to reduce effectively the fuel component in a short-circuited gas. When the fuel density in the fuel-containing gas itself is minimized, the fuel component introduced to the exhaust system as a short-circuited gas is reduced, but the fuel density in the fuel-containing gas remaining in the combustion chamber is also reduced, and the power output of the 2-cycle engine declines. To surmount this problem, a scheme has been devised wherein the pressure in the crankcase is set to 142 kPa or lower and a gas B with a low fuel density is introduced into the combustion chamber first, a gas A with a high fuel density is introduced into the combustion chamber subsequently, and the gas remaining in the combustion chamber is primarily A, and the gas short-circuited to the exhaust port is primarily B. However, when gases A and B mix in the combustion chamber, the effect of introducing gases A and B into the combustion chamber separately declines.